An NF (3°×2.25°) and WF (9°×6.75°) camera open at F/2.7 with a narrow-field focal length FNF equal to approximately 180 mm, wide-field focal length FWF equal to approximately 60 mm, fitted with a multiple quantum well IR3 (7.5-10 μm) matrix detector is known.
The photometric behavior of this camera is optimized for thermal infrared. Specifically:                the aperture diaphragm is materialized by the cold screen 3 of the detector 1 (represented in FIG. 1) protected by its window 2,        there is no vignetting because the only element limiting the beams is the aperture diaphragm.        
This camera is very compact. Specifically:                the path L of the beam along the optical axis between the entrance refractive surface and the focal plane (the detector 1) is less than FNF/1.2,        the section of the duct occupied by the optical beams is less than FNF/2.5 between a plane situated at FNF mm upstream of the entrance refractive surface and the focal plane.        
This compactness is obtained by virtue of:                a narrow-field entrance pupil situated in the vicinity of the front lens: this feature requires the presence of an IFP (Intermediate Focal Plane) in the optical combination,        a real wide-field entrance pupil, that is to say situated upstream of the front lens.        
The optical combination of this camera comprises the following refractive groups described with reference to FIG. 1:                a. a convergent front group G1 of focal length F, where F<FNF/2, therefore extremely open,        b. a divergent field-change group G2 that can move along the optical axis; this group is situated upstream of the IFP in NF configuration and downstream of the IFP in WF configuration,        c. a relay group G3 imaging the IFP on the focal plane of the detector.        
The group G1 is achromatized in the IR3 band by virtue of one of the conventional sequences Ge(+)/ZnSe(−) or Ge(+)/ZnS(−) or Ge(+)/DOE(+), with Ge for germanium, ZnSe for zinc selenide, ZnS for zinc sulfide, DOE for diffractive optical element, + for convergent and − for divergent.
The object of the invention is to be able to use such a camera and more generally an imaging system also in the IR2 band (3.5-5 μm) with a single detector. The 2 bands are however not used simultaneously: the useful band is selected by adjusting the polarization of the detector.
The existing camera, and more precisely the front group G1, has considerable chromatic aberration in IR2 which compromises its use in this spectral band.